If the air assist cylinder was hung, the clutch pedal would be stuck on the floor. Do you have a normal clutch pedal movement and pressure? Can you get someone to press on the pedal while you look at the linkage on the gearbox to see if everything is moving right? Are you sure you are getting it in gear properly? Has the reverse solenoid hung up at all or is it operating correctly? You can see the reverse solenoid and the top of the clutch linkage through the access panels in the floor at the back of the bus.

Just press the pedal without air if you don't have about 2 inches of free play on the pedal start checking beginning with the adjustment in the rear side door, the assist pivots and will hang in upwards position or the level position from time to time is what I find

There is a bunch of linkage and circuital adjustments associated with the air assist fwiw that is where I would start JMO they never get greased

Lots of ways for a clutch to fail but two of the most popular are for the friction lining to burn up and causes massive slipping. If it can't engage at all, you could still put it in gear but you'd still feel the clutch spring there. The other way (seen it on race cars but no idea on bus clutches) is for the center to rip out of the clutch disc so the engine is basically not connected to the input shaft any longer. Without more information from Steve it's hard to guess what's happening in this case.